Bandgap reference voltage circuits are used to supply a relatively constant voltage for electronic circuits, especially those using emitter coupled logic (ECL). For instance, a bandgap reference voltage circuit generates a reference voltage for logic circuits such as current sources and/or input reference voltages in ECL gates.
Widlar bandgap reference voltage circuits as well as reference voltage circuits employing operational amplifiers (op amps) are typically used in the prior art. An explanation of the problems associated with prior art reference voltage circuits follows with reference to FIGS. 1 and 2.
FIG. 1 illustrates a Widlar bandgap reference voltage circuit. Current source 2, which derives its current from the circuit power supply (not shown), is connected to the base of transistor 4 and the collector of transistor 14. The emitter of transistor 4 supplies collector current 20 through resistor 6 and collector current 22 through resistor 8 to the collectors of transistors 10 and 12, respectively. The reference voltage, Vref, is determined by the voltage across resistor 8 plus the voltage across the base-emitter junction of transistor 14, Vbe.sub.14. Neglecting the base current through transistor 12, current 22 is approximately equal to emitter current 24 through resistor 16. Since the voltage across resistor 16 is equal to the difference in the base-emitter voltages of transistors 10 and 12 or rather .DELTA.Vbe, the current through resistor 16 is .DELTA.Vbe/R16, where R16 is the value of resistor 16.
Neglecting base currents, the voltage drop across resistor 8 is simply R8.times..DELTA.Vbe/R16, where R8 is the value of resistor 8. Therefore, Vref is equal to Vbe.sub.14 +R8.times..DELTA.Vbe/R16. Many ECL devices require power supply operation ranges of 4.2 to 4.8 volts or 4.9 to 5.5 volts. The circuit described above and shown in FIG. 1 has one serious drawback in that the current from current source 2 is derived from the power supply and may vary with power supply voltage variations over a specified range. For many applications, variation of the reference voltage with a variation in the supply voltage over a specified range, is unsuitable for proper operation.
One possible solution in the prior art to curb reference voltage variation with respect to power supply variation is to provide a reference voltage circuit which includes an operational amplifier (op amp). A schematic drawing of this op amp reference circuit is illustrated in FIG. 2. FIG. 2 shows two diode configured transistors, 34 and 38 connected to the negative and positive input terminals, respectively, of op amp 40. Resistor 32 is connected to and between the negative terminal of op amp 40 and the collector of transistor 34. Current at node is fed back through resistors 30 (which is connected to resistor 32), and 32 and through resistor 36 which is connected to the collector of transistor 38. Assuming that base currents of transistors 34 and 38 are negligible and that the differential input of op am 40 is zero, i.e. .DELTA.V=0, then an expression for Vref is Vbel+KV.sub.T, where Vbel is the base-emitter voltage of transistor 38, K is a constant and V.sub.T is the electronvolt equivalent of the temperature. As observed, the expression for Vref shows some independence from voltage supply variation. However, implementation of the circuit illustrated in FIG. 2 requires an op amp with very precise components which add to the complexity and cost of the voltage reference band-gap circuit.